1. Field of Invention
The present invention relates to a wind power generation system. More particularly, the present invention relates to a wind power converter structure in the wind power generation system.
2. Description of Related Art
Currently, with continuously increasing rated power of a wind power turbine, the length of a wind wheel blade is gradually increased but the rotational speed of a rotor is gradually decreased. For example, in a wind power turbine with a rated power of 5 MW, the blade length is longer than 60 m, and the rated rotational speed of the rotor is about 10 rpm. When a power generator has two opposite poles, the gear ratio of a mechanical gearbox should be set to 150, such that the 5 MW wind power generator is directly connected with a power grid having a power frequency of 50 Hz in an AC manner. However, increasing the gear ratio of the mechanical gearbox will cause multiple challenges to designing and manufacturing the gearbox of the megawatt wind power turbine, because when the rated power of the wind power turbine and the gear ratio of the gearbox are increased, the dimension, weight and frictional wear of the wind power turbine are also increased. Thus, a full-power wind power converter is gradually adopted in the prior art for connecting the megawatt wind power turbine with the power grid in an AC-DC-AC manner, so as to solve the problems caused by continuously increasing gear ratio of the gearbox.
In particular, the full-power wind power converter is a back-to-back frequency conversion system formed by connecting two sets of power electronic converters via a DC link. The back-to-back frequency conversion system includes a converter at a power grid side (or referred to as a “grid-side converter”) and a converter at a power generator side (or referred to as a “generator-side converter”). The generator-side converter receives and transfers active power generated by an induction generator to the grid-side converter via the DC link. Furthermore, the generator-side converter further excites the induction generator through the stator terminal of the induction generator. Subsequently, the grid-side converter receives and transfers the active power transferred via the DC link to the power grid, i.e. enables voltages at two sides of the DC link to reach balance.
In the prior art, the full-power wind power converter is bigger with the increase of the capacity of the wind power generator. Taking the current mainstream power grade of 1-3 MW (in the future the power may be increased to 5-8 MW) as an example, there are two types of arrangement locations of the wind power converter in the entire wind power turbine. In one type, the wind power converter is arranged in a top part of a tower, and in the other type, the wind power converter is arranged in a bottom part of the tower. However, if the wind power converter is arranged in the top part of the tower, the space in the nacelle located at the top part of the tower is limited and the load design of the nacelle should also be enhanced. Once the wind power converter is out of order, a maintenance person has to climb to the top part of the tower for performing maintenance. Since the height of the tower is generally about 100 m, this type of arrangement location has not only a potential safety risk but also a high maintenance cost. Furthermore, if the wind power converter is arranged in the bottom part of the tower, although the maintenance person does not need to climb to the top part of the tower for performing maintenance, yet the cost of a cable connected between the wind power generator located in the top part of the tower and the wind power converter located in the bottom part of the tower is increased with the power increasing of the wind power generator. Especially for a three-phase power generator, longer transmission cables are required, thus not only increasing the purchase cost of cables but also the system installation cost of the wind power turbine.
In view of this, those in the industry are endeavoring to find ways to design a more appropriate wind power converter structure, which can not only balance the load design of the nacelle located on the top of the tower, but also ensure operation reliability of the wind power converter as well as reducing the installation cost of the transmission cable.